It is currently desirable to use moment-resistant structures in construction of new buildings because they offer fewer restrictions for design and more useful space, while adhering to building design codes. The moment-resistant structures must meet building code standards that require the moment-resistant connections to deflect and absorb energy resulting from earthquakes or high winds. A rigid joint is typically used to resist lateral forces by holding columns and beams fixed in relation to one another. As a result, the joints can become highly stressed during a seismic event. In a moment-resisting structure, the vertical and lateral loads are resisted by the bending strength of the beams and columns. Modern building codes require the strength of the columns and beams be proportioned to prevent column failure by allowing permanent deformation in the beam prior to any column failure. The greatest demand on the columns and beams occurs at and adjacent to the joints between columns and beams.
Moment-resistant frames are most often made of structural steel with bolted or welded joints. Bolted moment frames require plates welded to the ends of beams, that are in turn bolted to the column. Successful moment-resisting frames provide a ductile structure that will distort prior to failure and if properly detailed will not fail in a brittle manner. These frames develop their resistance to lateral forces through the flexural strength and continuity of the beam and column.
An acceptable moment-resisting beam column joint must remain rigid to the point of beam failure. Often, the Reduced Beam Section (RBS) is used to provide a “fuse” in the beam where failure occurs while the joint between the column and beam remains rigid. To accomplish this the joint must resist compression and tension forces produced by the bending in the beam at the beam flanges.
In the past most columns and beams have been “I” shaped members called wide flange sections. The top and bottom of the “I” section is the flange. Typically the beams frame into the column flange, which is the “strong axis” direction of the section. The beam flanges are usually welded to the flange(s) of the column. This configuration gives the column-beam joint great strength in one direction. To provide the same strength in the other direction, at 90 degrees from the first direction, some columns in the structure must be rotated or loads must be resisted in the “weak axis” direction of the column. This would require a stronger column to resist weak axis loads and use of configurations for weak axis column joints. Most small buildings require columns that must resist loads in each direction. This is a problem for “I” section columns in that much larger columns would be required to resist weak axis loads.
In contrast, Hollow Square Sections (HSS) have the same properties in each direction. Using hollow square tube sections for columns can make design and detailing essentially the same in each direction and the same column can be used for moment resisting connections in each direction. However, the HSS column presents a challenge in another way from the typical “I” section column in making the moment-resisting connection between the beam and the column.
A problem arises when assembling the beams to the columns where multiple beams and columns are required. With bolted moment frames using HSS columns, a through-bolt connection could be used to secure the beam end plate to the column. However, when multiple columns and beams are used to span a length of building, the use of through-bolts would necessitate aligning and securing multiple beams simultaneously to the columns. In the case of a linear span, the assembly process requires the holes of a first beam end plate to be aligned with the through-holes in a column and aligning the holes of a second, opposing beam plate with the column through-holes, then inserting the through bolts in each hole for tightening with nuts and washers. It would be necessary to install all the beams at each line and level simultaneously. This process could be slow, difficult, expensive or very impractical.
In the ongoing effort to improve building frame construction that better handles severe lateral loads, such as earthquake and high-wind loads, much attention has been focused on the manner in which upright columns and horizontal beams are connected. Attempts to addresses this issue include a column-beam interconnect with the ends of beams joined to columns using nodes of intersection and collar structures that surround the sides of the column as taught by Simmons et al. (U.S. Pat. No. 6,837,016). Other attempts include Okawa et al., (U.S. Pat. No. 5,410,847) who teaches a rod-like orthogonal metal connector provided in concrete structure members with junction hardware to connect a steel member to the structure member. Chen, (U.S. Pat. No. 5,595,040) teaches a beam-to-column connection for connecting an H-beam to a column surface, where the connection is defined at an end of the H-beam having a web plate and a pair of flange plates. Houghton (U.S. Pat. No. 6,138,427) teaches a moment resisting, beam-to-column connection, using two gusset plates attached to a column and extending along the sides of a beam and having connecting elements, where the connecting elements are bolted, riveted or welded to the beam along its longitudinal direction and to the gusset plates. Katayama et al. (U.S. Pat. No. 6,532,713) teaches a composite beam connected to a column by inserting a mortise pin into a bottom hole of a column and then inserting a locking pin into a through-hole of the mortise pin and the horizontal hole of the column such that joint of the composite beam and the column is firmly secured. Further, Briggs (U.S. Pat. No. 3,593,477) teaches a concrete beam reinforcement anchor embedded in the concrete, which has a plane surface in the side-face of the beam or column for bolting a beam thereto. Additionally, Sato et al. (U.S. Pat. No. 5,012,622) teach a solid concrete core thrust into a column, which then disallows the use through-bolt assembly and necessitates a clamping assembly that has limited utility.
These and other designs and systems have been used to make this connection but they are considered costly, less flexible or impracticable to build. Accordingly, there is a need to develop a system that allows for the less expensive assembly of the beams to the columns in moment frames. It would be considered an advance in the art using through bolts to connect beams to columns without the need for multiple beams to be installed at the same time, thus simplifying assembly for faster and less expensive construction.